Drug Absorption
Drugs must reach their targets selectively and controllably if their desired pharmacological activities are to be maximized. One approach to optimizing the activities of drugs is their controlled and sustained delivery into the systemic circulation. Orally administered drugs are generally absorbed in the intestine. Such drugs undergo first pass clearance by the liver and small intestine; that is, they are converted by the intestine and the liver to pharmacologically inactive metabolites and/or are secreted into bile by the liver, either as drug or as active metabolites. As a result, the amount of an orally administered drug actually entering the systemic circulation can be much less than the amount administered. To ensure that effective quantities of such a drug will enter the circulation and reach the targeted site(s) in the body, larger quantities than actually needed must be administered and often must be given in several smaller doses, rather than one dose. Orally administered drugs also typically have poor bioavailability. For example, they may be adversely affected by the pH and the enzymatic activity of the stomach and intestine and may be poorly dissolved in the stomach and intestinal fluids.
There have been numerous attempts to address these problems and to improve the bioavailability of orally administered drugs. The efficacy of some drugs given orally has been improved by administering them with a triglyceride or neutral fat. Such fats represent an environment that is compatible with lipophilic drugs, i.e. they exhibit low aqueous solubility. Fats also enhance the stability of drugs which are unstable in the stomach and intestine. The end products of fat digestion are absorbed by the villi of the intestinal mucosa into a lymphatic vessel, the central lacteal; absorption occurs within a region of the intestine in which limited drug metabolism occurs. The absorbed fat is transported through the thoracic duct, the major lymphatic channel and is subsequently emptied into the blood; it is not carried in the portal blood, which goes to the liver, where first pass metabolism of drugs occurs.
The absorption of griseofulvin has been shown to be enhanced if the drug is co-administered with a high fat content meal or in an oil and water emulsion. Crounse, R. G., Journal of Investigative Dermatology, 37:529 (1961); Carrigan, P. J. and Bates, T. R., Journal of Pharmacological Science, 62:1476 (1973). If the hormone testosterone undecanoate is administered in a peanut oil solution, it is more biologically active than if it is administered in an aqueous micro-crystalline suspension. Coert, A. J. et al., Acta Endocrinol, 79: 789, (1975); Hirschhauser, C. et al., Acta Endocrinol., 80:179. (1975). This effect is presumed to be due to absorption of the steroid via the thoracic lymph rather than the portal blood; in this way, first pass clearance by the liver is avoided.
Cholesterol, its esters and triglyceride constituents (e.g., fatty acids and monoglycerides) are absorbed via the thoracic lymph. The effects of some of these compounds, alone or in the presence of bile salts, upon absorption of some orally administered drugs have been evaluated. For example, oral administration of ubidecarenone, which is used for treating hypertension, in a mixture containing fatty acids having 12 to 18 carbon atoms and monoglycerides containing such fatty acids, resulted in somewhat greater absorption of the ubidecarenone than occurred after oral administration of the drug alone (8.3% v. 2.3%). Taki, K. and Takahira, H., U.S. Pat. No. 4,325,942. (1982). If the steroid progesterone is administered orally in combination with cholesterol or its esters, good sustained biological activity can be obtained. This is believed to be due to the absorption of progesterone via the thoracic lymph and not via the portal circulation. Kincl, F. A., Proceedings of the 6th International Congress of Pharmacology, 5:105, (1975).
Yesair has evaluated the effect of fatty acids having 12 to 18 carbon atoms, monoglycerides of these fatty acids, and bile salts on the absorption of orally administered estradiol, which is an estrogenic hormone. yesair, D. W., PCT WO 83/00294 (1983). The mole ratio of fatty acids: monoglycerides: bile salts evaluated ranged from 10:1:1, 1:1:10 or 1:10:1. The preferred ratio is stated to be 2:1:2, which is similar to the micellar composition resulting from the enzymatic digestion of triglycerides in the intestine, which occurs in the presence of bile salts and calcium ions. When excess bile salts are present, estradiol incorporated into the 2:1:2 composition can migrate or partition into a bile salt-enriched micellar solution. This migration or partitioning of estradiol occurred prior to absorption of the drug, as shown by the fact that the initial concentrations in plasma of estradiol are initially greater than those in lymph. In addition, about 25-50% of the estradiol administered in the composition was co-absorbed with the lipid constituents and entered the systemic circulation via the thoracic lymph.
The presence of bile salts, which are absorbed in the ileum (and not in the jejunum, as is most fat) compromised the co-absorption of estradiol with fat by enhancing the migration of the drug from fat to the bile salt micelle. Phosphatidyl choline was used in an effort to maintain the estradiol within the micellar composition in which fatty acids:-monoglycerides:bile salts occurred in a 2:1:2 molar ratio. In the presence of excess bile salts, about 60% of the estradiol incorporated into the 2:1:2 micellar composition remained associated with it when phosphatidyl choline was not present. Under the same conditions, about 70-75% of the estradiol remained in the composition when phosphatidyl choline was used. Addition of phosphatidyl choline for this purpose, however, results in an increased size of the delivery system. Size is an important parameter in the absorption of lipid micelles and this effect of phosphatidyl choline might interfere with co-absorption of the drug with the lipids. In addition, excess phosphatidyl choline has been shown to reduce lipid absorption. Ammon, H. V. et al., Lipids, 14:395 (1979); Clark, S. B., Gastrointestinal Physiology, 4: E183 (1978).
Others have also described the effects of the presence of bile salts in lipid formulations used for co-absorption of drugs. Wilson, T. H., In: Intestinal Absorption, Saunders, (1962); Lack, L. and Weiner, I. M., American Journal of Physiology, 240:313, (1961); H. V. Ammon et al, Lipids, 14: 395, (1979). For example, little difference in the absorption of 5-fluorouracil (5FU) in the stomach or small intestine was evident when the 5FU was administered alone or in a mono-olein/sodium taurocholate mixed micelle formulation. 5FU absorption in the large intestine was greater when the drug was administered in the formulation than when it was administered alone. Streptomycin is poorly absorbed from the intestine. Muranushi and co-workers report that mixed micelles, composed of bile salts, monoolein or unsaturated fatty acids, did not improve the absorption of streptomycin from the small intestine but markedly enhanced the absorption from the large intestine. The enhancement in the large intestine was attributed mostly to the alteration of the mucosal membrane permeability by mono-olein or unsaturated fatty acids. In contrast, mixed micelles of bile salts and saturated fatty acids produced only a small enhancement in streptomycin absorption even from the large intestine. Muranushi, N. et al., Journal of Pharmaceutics, 4: 271 (1980). Taniguchi et al. report that monoolein/taurocholate or oleic acid/taurocholate promotes the absorption of heparin, which is poorly absorbed when administered alone. Taniguchi, K. et al., International Journal of Pharmaceutics, 4:219 (1980). Absorption of heparin from the large intestine was twice that which occurred from the small intestine. The concentration of heparin in the mixed micelle to produce the potentiation in the large intestine was approximately one-fourth that required in the small intestine.
In U.S. Pat. No. 4,156,719, Sezoski and Muranishi describe a micelle solution for rectal administration of water-soluble drugs that are poorly absorbed. The composition consists of fatty acids having 6-18 carbons, and/or mono-or diglycerides having the same type of fatty acids; a bile salt or other non-ionic surface active agent; and water. A lysophosphatidyl choline moiety can be substituted for the fatty acids and mono- or diglycerides. Absorption of streptomycin and gentamycin from the rectum and large intestine is reported to be comparable when the drug is administered in a bile salt: mixed lipid micelle. Similar formulations were not effective in increasing absorption in the duodenum. Muranushi, S. et al., International Journal of Pharmaceutics, 2:101 (1979). Absorption of the two drugs via the rectum and large intestine was markedly greater than that of a comparable dose administered duodenally, even when the mixed lipid micelle concentration administered duodenally was four times that administered via the other routes.
At the present time, there is a need for a more effective method of enhancing absorption of orally administered drugs from the small intestine. Much of a drug administered orally never reaches the targeted sites, either because the drug is poorly absorbed or because after it is absorbed, much is removed by the liver. If a more effective means were available to enhance the absorption of a drug, therapy would be more efficient because a smaller quantity of the drug would need to be administered to ensure that the desired amount would reach the target sites.